1. Field of the Invention
The invention relates to protective coatings for substrates, including high moisture edible substrates. More particularly, the invention relates to coating compositions, and to a novel method of forming a protective coating on substrates from emulsions and suspensions of proteins and hydrophobic materials.
2. Description of the Art
Protective coatings are useful to protect a base substrate from the deteriorative effects of gases such as oxygen, from detrimental environmental conditions such as high or low humidity, and from factors which cause biological degradation. Substrates, especially those containing moisture and which have the propensity to lose moisture through evaporation, e.g., agricultural products, are particularly vulnerable to loss of quality of the substrate over time. Edible agricultural products such as fresh, frozen, whole or cut, fruits and vegetables, meat, fish, eggs, grains, nuts, and inedible agricultural products such as living plants, plant products, and ornamentals are subject to loss of quality over time from moisture loss, enhanced respiration and senescence, and browning and oxidative degradation. Other deterimental effects to agricultural products can result from microbial attack and moisture penetration.
Specifically, many edible products and plant materials such as fruits, vegetables, grains, eggs, plants, and ornamentals, continue to maintain life function after harvest. They continue to respire until decay, consuming oxygen and producing carbon dioxide. The time these products are available in a fresh and attractive form can be extended, if respiration can be slowed down by limiting availability of oxygen or if the carbon dioxide level can be maintained at an optimum level.
Further, many edible products and plant materials have components which are vulnerable to oxidation, with resultant loss in quality, as oxygen diffuses into the tissue of the food or plant material. For example, fresh and frozen fish, frozen fruits and vegetables, nuts, and ornamentals have a limited shelf-life which is due to such oxidation. The time these products are available in a quality form can be extended, if oxidation can be slowed down by limiting diffusion of oxygen into the product.
Many substrates such as edible products and plant materials have a high moisture content and are vulnerable to quality loss as they lose their moisture to the air. In particular, fresh fruits and vegetables, eggs, fish, living or cut trees, plants, and ornamentals, for example, have a limited shelf-life which is due in part to loss of moisture to the atmosphere. Products which have peels, skins, or shells tend to have retarded moisture loss; but over a period of time enough moisture can be lost to lower the product quality to the point of product rejection.
Substrates which are high in moisture content and have high moisture at the surface are particularly vulnerable to loss of quality due to moisture loss. Examples are fruits and vegetables and other foods, and plant products which have exposed tissue surfaces created by peeling, cutting, etc. such as peeled and/or sliced apples, sliced tomatoes, peeled eggs, fish filets, and cut-stem flowers. Because their natural skins, peels, and shells, which normally act to retard moisture loss have been removed, these products lose their quality quickly.
Edible films have been proposed for use on foods to control respiration, reduce oxidation, or limit moisture loss. (See: J. J. Kester and O. R. Fennema, "Edible Films and Coatings: A Review," Food Technology 40: 47-59 (1986) and S. Guilbert, "Technology and Application of Edible Protective Films," Food Packaging and Preservation Theory and Practice, Ed. M. Mathlouthi, Elsevier Applied Science Publishing Co., London, England (1986), pages 371-394). Coatings for edible products include wax emulsions (U.S. Pat. No. 2,560,820 to Recker and U.S. Pat. No. 2,703,760 to Cunning); coatings of natural materials including milk solids (U.S. Pat. No. 2,282,801 to Musher), lecithin (U.S. Pat. No. 2,470,281 to Allingham and U.S. Pat. No. 3,451,826 to Mulder), algin and a gelling mixture (U.S. Pat. No. 4,504,502 to Earle and McKee), protein (U.S. Pat. No. 4,344,971 to Garbutt), dispersions of a hydrophilic film former and an edible fat (U.S. Pat. No. 3,323,922 to Durst), and dispersions of hydrophobic materials in aqueous solutions of water-soluble high polymers (U.S. Pat. No. 3,997,674 to Ukai et al.).
In most cases, the foods to be protected have been whole, uncut foods with peels and skins in place, for example, whole unpeeled apples, whole tomatoes, and unpeeled eggs. In these cases, an edible film is formed on, and supplements, a surface which has low-moisture content, e.g., an apple peel or an egg shell, and already acts as a gas (oxygen or carbon dioxide) and moisture barrier. The film is not in direct contact with a high-moisture surface. Suitable coatings for high moisture substrates having high moisture at the surface such as peeled fruits and vegetables and eggs, have not been developed heretofore.